In recent years, in the medical field, as an output means of diagnostic images such as X-ray CT and MRI, an image forming method has been employed in which exposure is performed by scanning laser beams onto photosensitive materials. Thus, these photosensitive materials are subjected to image exposure by scanning laser beams. Thereafter, image exposed materials are processed employing a wet photographic processing and employed for diagnosis.
When photosensitive materials are subjected to wet processing, effluent has resulted in problems in view of environmental protection. In recent years, additionally, in view of space saving, it has been strongly demanded to reduce the amount of spent processing effluent.
Consequently, techniques related to photothermographic materials have been sought which can be efficiently exposed employing a laser imager, and can also form clear black images at high resolution.
As described, for example, in U.S. Pat. Nos. 3,152,904 and 3,487,075, and D. Morgan, “Dry Silver Photographic Materials” (Handbook of Imaging Materials, Marcel Dekker, Inc., page 48, 1991), known as realization of such techniques are photothermographic materials which comprise a support having thereon organic silver salts, photosensitive silver halide grains and reducing agents.
Listed as essential constituting components in the aforesaid photothermographic materials and thermally developable materials are organic silver salts. Organic silver salts are insoluble in organic solvents. Consequently, when they are employed as a photosensitive material, it is required to coat a dispersion, which is prepared by dispersing organic silver salts containing silver halides into dispersing binders having layer forming property. Specifically employed as the most suitable binders having layer forming property are polyvinyl acetal resins. However, commonly available polyvinyl acetal resins are comprised of various compositions. Further, due to the production methods, a small amount of impurities are incorporated. Such impurities occasionally result in degradation of image characteristics after coating such as fog formation, poor image tone, or insufficient storage stability of unexposed film.
In order to overcome the above-mentioned drawbacks, addition of components such as antifoggants or toners, which improve photographic image characteristic, is knows in the art. Patent Documents 1 and 2 disclose that it is also possible to perform improvements depending on compositions and characteristics of binders in photosensitive materials.
However, in the case in which the methods described in the above-mentioned Patent Documents 1 and 2 were employed, image characteristics were improved. However, it was discovered that other problems occurred in which the particle size distribution of dispersed organic silver salt particles was broadened and silver halides readily aggregated, whereby the maximum density of the resulting photosensitive materials became insufficient.
It is ideal that in photothermographic materials, photosensitive silver halides as a photocatalyst, reducible organic silver salts, and thermally activated reducing agents are arranged in a desired balance in a coating. In order to achieve the more desired image characteristics, a more desired organic silver salt dispersion and the production method thereof are needed. Uniformly arranging the organic silver salts may be performed by enhancing dispersibility of the organic silver salts. The inventors of the present invention paid close attention to the production process of organic silver salts and investigated ways of improving dispersibility. As a result, it was discovered that it was possible to prepare organic silver salts of a relatively small particle diameter, which especially exhibited desired dispersibility. Patent Documents 3 and 4 describe organic silver salts comprised of sodium salts or potassium salts. However, the inventors of the present invention discovered that by improving both organic silver salts and dispersing binders, it was possible to prepare an organic silver salt dispersion which exhibited better dispersibility than that by methods known in the prior art, and a photothermographic material which exhibited excellent image characteristics.
(Patent Document 1) JP-A No. 2000-235242
(Patent Document 2) JP-A No. 2002-355543
(Patent Document 3) JP-A No. 5-53239
(Patent Document 4) JP-A No. 8-234358